Rounding a spinner on a British Spitfire
Here are the steps I would take to model that:

1: construct a base set of vertices from one side of the rounded top, like so:



Add a screw modifier: (make sure it is lined up to the axis for this part).



Then finally add a subidivision surface modifier to make it higher poly:



I hope that is what you are looking for. You can just adjust the vertices (or line them up to your image) to get the curve you want. When you are done, you can apply the modifiers and put it on your plane.